The present invention relates to a method of manufacturing an article by superplastic forming and diffusion bonding.
It is known to manufacture hollow metallic articles by superplastic forming and diffusion bonding metal workpieces. These metal workpieces include elementary metal, metal alloys, intermetallic materials and metal matrix composites.
The superplastic forming and diffusion process may be used to produce contoured articles for example fan blades, or fan duct outlet guide vanes, for gas turbine engines by superplastically, or hot forming, an integral structure formed by the diffusion bonding process.
A procedure for manufacturing an article by superplastic forming and diffusion bonding is disclosed in our European patent EP0568201B. In EP0568201B the integral structure formed by the diffusion bonding process is twisted before the integral structure is superplastically formed. Additionally the integral structure is hot creep formed in the superplastic forming dies.
A procedure for manufacturing an article by superplastic forming and diffusion bonding is disclosed in our UK patent GB2269555B. In GB2269555B the surfaces of two of the metal workpieces are machined to produce a predetermined mass distribution in the metal workpieces. The opposite surfaces of the two metal workpieces are maintained flat and the flat surfaces are diffusion bonded together or are bonded to opposite surfaces of a third metal workpiece. The integral structure is hot creep formed in the superplastic forming dies.
However, there is a problem with this manufacturing process. The superplastic, or hot, forming of the integral structure, after diffusion bonding, in the dies results in creasing of the metal workpiece hot formed on the convex surface of the superplastic forming die. This is because the metal workpiece hot formed on the convex surface of the superplastic forming die is under compression. This results in non-conformance of the article with desired dimensions and may result in the scrapping of the article, and the resulting waist of material, time and money.
Accordingly the present invention seeks to provide a novel method of manufacturing an article by superplastic forming and diffusion bonding which overcomes the above mentioned problems.
Accordingly the present invention provides a method of manufacturing an article by superplastic forming and diffusion bonding at least two metal workpieces comprising the steps of:
(a) forming a first metal workpiece which has a flat surface,
(b) forming a second metal workpiece which has a flat surface, the second metal workpieces being thicker than the first metal workpiece, the first and second metal workpieces defining the outer profile of the finished article,
(c) machining the second metal workpiece on a surface opposite to the flat surface to give a predetermined mass distribution in the second metal workpiece,
(d) applying a stop off material to prevent diffusion bonding to preselected areas of at least one of the flat surfaces of at least one of the at least two workpieces,
(e) assembling the at least two metal workpieces into a stack relative to each other so that the flat surfaces are in mating abutment,
(f) applying heat and pressure across the thickness of the at least two metal workpieces to diffusion bond the at least two metal workpieces together in areas other than the preselected areas to form an integral structure,
(g) placing the integral structure in a hot creep forming die, the die having a convex surface, the integral structure being placed in the die such that the first metal workpiece faces the convex surface of the die, heating the integral structure while it is within the die to cause the integral structure to be hot creep formed on the convex surface of the die,
(h) placing the integral structure in a superplastic forming dies heating the integral structure and internally pressurising the integral structure while it is within the die to cause the preselected area of at least the second metal workpiece to be hot formed to produce a hollow article of predetermined shape.
After twisting the integral structure and before superplastic forming the integral structure, the contoured integral structure may be internally pressurised to break the adhesive bond between the stop off material and the at least one workpiece in the preselected area.
Preferably after internally pressurising the integral structure to break the adhesive bond and before internally pressurising the integral structure to superplastically form at least one metal workpiece, the interior of the integral structure is sequentially evacuated and supplied with inert gas to remove oxygen from the interior of the integral structure.
Preferably after diffusion bonding the stack of workpieces and before superplastically forming the integral structure, the integral structure is internally pressurised to break the adhesive bond between the stop off material and the at least one workpiece in the preselected area.
Preferably after internally pressurising the integral structure to break the adhesive bond and before internally pressurising the integral structure to superplastically form at least one metal workpiece, the interior of the integral structure is sequentially evacuated and supplied with inert gas to remove oxygen from the interior of the integral structure.
Preferably after the metal workpieces are arranged in a stack and before the metal workpieces are diffusion bonded together to form an integral structure, the edges of the metal workpieces are sealed.
Preferably the edges of the metal workpieces are welded together.
Preferably where the metal workpieces are made of a titanium alloy, the metal workpieces are heated to a temperature equal to, or greater than, 850xc2x0 C. and the pressure applied is equal to, or greater than, 20xc3x97105 Nmxe2x88x922 to diffusion bond the workpieces together to form an integral structure.
Preferably the metal workpieces are heated to a temperature between 900xc2x0 C. and 950xc2x0 C. and the pressure applied is between 20xc3x97105 Nmxe2x88x922 and 30xc3x97105 Nmxe2x88x922.
Preferably the integral structure is heated to a temperature equal to, or greater than, 850xc2x0 C. to superplastically form the integral structure.
Preferably the integral structure is heated to a temperature between 900xc2x0 and 950xc2x0 C.
Preferably the integral structure is hot creep formed at a temperature equal to, or greater than, 740xc2x0 C.
Preferably step (c) comprises milling, electrochemical machining chemical machining, or electrodischarge machining.